The digital PSTN (Public Switched Telephony Network), intended for primarily communicating voice information between two telephone sets, is a synchronous network, see FIG. 1. The A/D and D/A converters in the two line circuits at the ends of a connection, i.e. at the subscriber lines to the two telephone sets, use “the same” 8 kHz sample clock signal, see FIG. 2.
If a packet network using e.g. Ethernet is used for transport of the voice information over some portion of the path between the two telephone sets, see FIG. 3, no common reference clock signal is available that could be used by the A/D and D/A converters. The voice samples are transported in packets, the packets used also called frames according to Ethernet. The arrival times of the packets vary dependent on delay variations in the packet network. Then, jitter buffers at the receive ends are needed, see FIG. 4, to handle the delay variations. If clock signals that are not synchronized with each other are used in the A/D and D/A converters at the ends of an established connection the jitter buffers will sooner or later overflow or be empty. Voice samples have to be deleted or inserted to avoid this, where this correction involving deleting/inserting voice samples is usually called a slip. If the slip is inserted during a silence period of the voice communication it will not be detectable to the person receiving the voice message.
However, for example some devices like ordinary telephone modems for communicating data or facsimile are sensitive to slips and clock jitter. Hence, many modems will retrain when a slip occurs. Also DTMF signals are disturbed by a slip.
POTS traffic, i.e. usual voice information, on a physical subscriber line or link also carrying an ADSL (Asymmetric Digital Subscriber Line) for communicating digital information is usually transferred in an own frequency band, obtained by splitting the available frequency band using analog splitting filters. The ADSL uses a band of frequencies from 25 kHz up to 1.1 MHz, or up to 2.2 MHz for the standard ADSL2+, and the POTS uses the normal POTS frequencies.
In VoDSL the POTS instead sends voice information coded as digital data within the ADSL payload. Then, the POTS CODEC (PCM Coder Decoder) is situated at the subscriber side of the ADSL link. The POTS CODEC needs an 8 kHz reference clock signal, such reference clock signals herein called simply “clock signals” or “clocks”.
Advantages of VoDSL include that if POTS uses the digital band, the normal POTS frequencies can be used for data and that no frequency splitting filters are needed.
For providing a reference clock for decoding voice information at the receive end of a communication path including a packet transport network a free running oscillator can be used. However, the expected accuracy of a low cost oscillator is in the order of 100 ppm, giving a slip-rate of one slip on 104 samples, i.e. one every 1.25 second. This is not acceptable for a high quality POTS system. In IP networks the network timing protocol (NTP) is used to synchronize nodes and it can be used for timing recovery, see published International patent applications Nos. WO 00/42728 and 02/39630 for Telefonaktiebolaget L M Ericsson and corresponding U.S. Pat. Nos. 6,532,274 and 6,711,411, inventor Stefano Ruffini. The standard IEEE 1588 is a level 2 method (Ethernet) for providing a clock signal. It requires Ethernet all the way, i.e. no routers can be connected in a connection path.
Various methods exist for generating a clock signal, such as from received packets, some of them requiring specially designed packets or non-standard Ethernet equipment. There is a need for methods that have a high performance and can use the existing packet streams in an Ethernet transport network.
“Adaptive clock” is a generic name for methods that regenerate a clock from packet timing. Some of these methods are point-to-point methods that work transparently through different sorts of network equipment like switches and routers. The quality of the regenerated clock is very much dependent on the variation of the delays of the packets. In e.g. a switch a packet carrying voice information might have to wait until a long data packet is transmitted. The time that the packet has to wait is proportional to the length of the data packet and the speed, e.g. in bits/s, of the link. The maximum length of Ethernet packets is 1500 bytes. In a 100 Mbit/s Ethernet link this corresponds to 120 μs and in Gigabit Ethernet it corresponds 12 μs. In an ADSL link the speed is much lower, e.g. 0.5-8 Mbit/s. Thus, e.g. a speed of 0.5 Mbit/s corresponds to 24 ms for an Ethernet packet of maximum length. Obviously, in that case the delays due to long data packets are much higher. This means that the delay variation of packets after a DSL link is increased and that adaptive clock methods over the ADSL link are less useful.
In the ADSL standard ITU-T G.992.1 the transfer of an 8 kHz timing reference signal is described. In a voice over DSL case the reference clock, available in the DSLAM, can be connected to the NTR (Network Timing Reference) unit and transferred to the customer. ADSL is originally designed for an ATM interface and in that case an 8 kHz reference signal is usually available from the ATM network.
In the NTR units a phase difference between the 8 kHz clock and the ADSL sample clock, that is obtained by dividing a transmit clock of 2.208 MHz by 276 to obtain the 8 kHz signal, see FIG. 5, is produced and a value thereof is transmitted to the customer.
In the case where the DSLAM (DSL Access Multiplexer) used in the uplink end of an ADSL is only connected via Ethernet such a reference clock in not available which is the case considered herein.